I'm building a site for my org that generates templates for users

I am able to copy a chosen template to the user's clipboard like so:

function copyTemplateText(){
    let text_to_copy = document.getElementById("templateText").innerHTML;

    if (!navigator.clipboard){
        // Pass
    } else{
        navigator.clipboard.writeText(text_to_copy).then(
          function(){
            console.log('copied'); // success 
          })
        .catch(
           function() {
            console.log('not copied');// fail 
        });
    }

This template would need to be pasted into the text area of an external website. Granted, users just need to CTRL+V and be done with it, but I was wondering if this can be done automatically. How can I pass a function from my site, to this new one? Is this possible?

Pseudo Code:

copyTemplateText('..runs');
let pasteHere = document.getElementByID('textBox');
pasteHere.innerText = What's in the clipboard;

I tried doing it with sessionStorage:

navigator.clipboard.readText().then(clipText =>
    sessionStorage.setItem("clipboard", clipText));
let pasteHere = document.getElementById("textBox");
pasteHere.innerText = sessionStorage.getItem("clipboard");

But that doesn't work. I get:

Uncaught TypeError: Cannot set properties of null (setting 'innerText || innerHTML')
    at HTMLAnchorElement.pasteTicketTemplate
Uncaught (in promise) DOMException: Document is not focused.

Is what I'm trying to do feasible? If so, how would I go about this?

Thanks a bunch!